For surveying purposes, the instrument mounted atop the tripod must be precisely placed directly above a particular point on a ground surface. In the past, the method of locating the instrument above the particular point was accomplished in one of two ways:
(1) a single pole with a point at one end was pushed into the ground by the surveyor at the predetermined point chosen. The pole was thereafter held in the vertical position by the assistant, who relied on a bubble level to indicate to him that the pole was in a correct vertical attitude. This method tends to introduce error into surveying, because the vertical attitude of the survey pole cannot be precisely maintained. It also wastes valuable manhours whenever the survey is extensive. One person has to stay with the pole to keep it erect and aligned. Ideally the instrument or other object attached to the top of the pole will be directly above the predetermined point.
(2) the other method for locating the instrument above the predetermined point involved a tripod and a plumbline. The plumb and line are hung from the correct point on the apex of the tripod. The tripod is thereafter adjusted and moved around until the tip of the plumb is directly above the predetermined point.